Communication systems that utilize coded communication signals are known in the art. One such system is a direct sequence code division multiple access (DS-CDMA) cellular communication system, such as that set forth in the Telecommunications Industry Association/Electronic Industries Association Interim Standard 95 (TIA/EIA IS-95), hereinafter referred to as IS-95. In accordance with IS-95, the coded communication signals used in the DS-CDMA system comprise DS-CDMA signals that are transmitted in a common 1.25 MHz bandwidth to the base sites of the system from communication units, such as mobile or portable radiotelephones, that are communicating in the coverage areas of the base sites. Each DS-CDMA signal includes, inter alia, a pseudo-noise (PN) sequence associated with a particular base site and an identification number of a communicating communication unit.
During a typical communication, the communicating communication unit often travels within the coverage area of the base site that is supporting the communication. Such movement typically results in fading of the communication signal transmitted from the communication unit to the base site due to multipath propagation of the transmitted signal. As is known, multipath propagation results from reflections of the transmitted signal off of nearby scatterers, such as buildings or large stones. These reflections produce replicas of the originally transmitted signal that arrive at the base site at various times depending on the effective propagation distances traveled by the replicas. The originally transmitted signal and the multipath replicas are typically referred to as multipath signals of the originally transmitted signal.
In a DS-CDMA system, such as that described by IS-95, multipath propagation typically results in multipath signals from each transmitting communication unit arriving at the base site at substantially the same time, or at least within a common time interval. The IS-95 system divides the DS-CDMA signals into 20 millisecond (ms) frames that contain sixteen power control groups. Each power control group is further divided into six modulation-or so-called Walsh-symbol time intervals.- Each Walsh symbol time interval is approximately 208 microseconds. Thus, with this frame configuration, several multipath signals from each transmitting communication unit might arrive at the base site during one of the Walsh symbol time intervals. Each Walsh symbol time interval generally contains one Walsh symbol of digital information as is known in the art.
To demodulate the transmissions from each communication unit, the base site receiver must first identify the multipath signals from each communication unit and then select the best multipath signals to demodulate. To identify a particular multipath signal, the base site initially receives and stores the information contained in one Walsh symbol. The base site then generates its corresponding PN sequence and correlates it to the PN sequence contained in the signal received at an initial time offset within the Walsh symbol to produce a correlation energy. The initial time offset is typically selected based on the theoretical minimum distance between a communication unit and the base site.
Upon obtaining the first correlation energy, the base site advances to the same offset in the next Walsh symbol and calculates a corresponding correlation energy. This advance and calculate, or search, process continues for a select number of Walsh symbols, typically up to the number contained in a power control group. The number of Walsh symbols to which a particular offset is applied is dependent upon whether the signal being searched for is a traffic signal (i.e., voice or data) or a preamble signal used during the registration of a communication unit in the DS-CDMA system. When the signal being searched for is a traffic signal, the offset is applied to all six Walsh symbols in the power control group; whereas, when the signal being searched for is a preamble signal, the offset is only applied to two Walsh symbols.
Upon applying the initial offset to the appropriate number of Walsh symbols, the base site changes the offset and repeats the procedure until it locates one or more multipath signals that have been transmitted from a communication unit within its service coverage area. Depending on the initial offset and the propagation distances traversed by the communication unit's transmission, the base site typically has to iterate through several offsets before obtaining multipath signals to demodulate. Thus, this search process can often result in high effective demodulation times (i.e., the time to search and subsequently demodulate), especially for received traffic signals. The high effective demodulation times impose an undesired upper limit on the bit energy to noise ratio performance required to maintain a particular received bit error rate. Such a limit prevents the DS-CDMA system from increasing its capacity without degrading the base site's received bit error rate (i.e., signal quality).
Therefore, a need exists for a method and apparatus that identify a particular coded communication signal from a plurality of received coded communication signals, while significantly reducing the search time required to perform the identification. Further, such a method and apparatus that improves the received bit energy to noise ratio for a given bit error rate would be an improvement over the prior art.